1. Field of the invention
This invention relates to a microwave integrated circuit mixer (MIC mixer) using a strip line or a microstrip line, etc. for outputting an IF signal with a frequency equal to the difference between the frequencies of the input radio frequency signal and a local oscillation signal (LO signal).
2. Description of a Prior Art
FIG. 1 shows a typical example of a prior art MIC mixer configuration. In the FIG. 1 circuit, a radio frequency signal (hereinafter RF signal) fed through an RF input terminal 1 propagates on a main line 2 and is supplied to a mixer diode 3, i.e., a frequency mixing element. A local oscillation signal (hereinafter LO signal) generated in a microwave oscillator 4 is given to the mixer diode 3 after passing through a band-pass filter (BPF) 5 for the LO signal which is coupled to the main line 2 at the respective high frequencies and passes only the LO signal selectively. Then the output of the mixer diode 3 is supplied to an IF signal output terminal 7 through a low-pass filter 6 which passes only the IF signal, and an end A of the mixer diode 3 is short-circuited for the RF signal and for the LO signal by a high frequency short-circuiting technique where a 1/4 wavelength transmission line is used. In the conventional apparatus of FIG. 1, the input terminal 1 is connected to the main line 2 through an IF interruption circuit 8 which has a passing characteristic for the input RF signal, but works as an open circuit impedance for the IF signal and is provided on the main line 2 at a distance of 1/4 wavelength of the IF signal (.perspectiveto.1/4.lambda..sub.if) from the mixer diode 3. Accordingly, the input side terminal "B" of the mixer diode 3 becomes short-circuited to a ground in the IF signal frequency. In the microwave oscillator 4, a high frequency use FET 21 is provided in a manner that drain terminal 22 thereof is connected to a strip line 23 which has 1/4 wavelength of the oscillation frequency, and the strip line 23 is open ended. The gate terminal 24 of the FET 21 is connected to one end of a strip line 25, and the other end of the strip line 25 is ground through a dummy resistor 26. A dielectric resonator 27 is disposed so as to be coupled with the strip line 25. The source terminal 28 of FET 21 is connected to one end of a strip line 29, the other end of which is grounded through a dummy resistor 19, so that the output of the oscillator which oscillates at a resonance frequency of the dielectric resonator 27 is taken out from the strip line 29 and fed to the mixer diode 3 through a filter circuit 29+5 and the main line 2. A low-pass filter 30 for feeding a bias current consists of a high impedance line part and a low impedance line part. The low impedance line part of the low-pass filter 30 is connected through a resistor 31 to a capacitor 32, and a bias power source is fed through a connection point between the resistor 31 and the capacitor 32. As the IF interruption circuit 8, a band-pass filter which has a small insertion loss for RF signals and a wide pass-band, and in actual practice may be an inter-digital type direct-current block where two open-ended strip lines are parallel-coupled over a length nearly equal to 1/4 wavelength (.lambda.s/4) of the RF signal from the open end as shown in FIG. 2, can be used. The capacitance due to a gap between the two open-ended strip-lines parallel-coupled is, for example, several 0.01 pF when the frequency of a RF signal is 12 GHz, and accordingly, the impedance of the gap capacitance becomes several k.OMEGA. when the IF frequency is 1 GHz, and the impedance can be ragarded as almost an open-circuit impedance. However, if a resonance circuit having a resonance characteristic in a frequency range of the IF signal is constituted in a circuit to be connected to the RF input terminal 1, then the gap capacitance of the IF interruption circuit 8 will work as a coupling capacitance, and accordingly, an impedance seen from the terminal "B" of the mixer diode 3 towards the IF interruption circuit 8 begin to have resonance characteristics, and the terminal "B" of the mixer diode 3 is not short-circuited to a ground around the resonance frequency.
Accordingly, the output impedance for the IF frequency at the output terminal 7 is greatly influenced around the resonance frequency, and the condition of the output matching at the IF signal is greatly influenced. As a summary, the frequency spectral characteristic of the mixer circuit begins to display a steep defect phenomenon, which is a fatal defect of the mixer circuit.
Furthermore, in a microwave oscillator 4, the gain of the transistor increases generally as the frequency becomes lower, and accordingly, a spurious oscillation is likely to be produced. In generally, in FIG. 1, provided that a reflection coefficient seen from the gate 24 of FET 21 towards FET 21 is S.sub.11 and a reflection coefficient seen from the gate terminal 24 towards the dummy resistor 26 is .GAMMA..sub.R, then the oscillation is carried out when the following condition is fulfilled: EQU S.sub.11 .times..GAMMA..sub.R =1 (1).
Accordingly, when the resonance circuit is set in a manner that the reflection coefficient .GAMMA..sub.R is .vertline..GAMMA..sub.R .vertline..perspectiveto.1 only for the frequency around the resonance frequency of the dielectric resonator 27, and .vertline..GAMMA..sub.R .vertline..perspectiveto.0 for the frequency other than the resonance frequency, then the microwave oscillator 4 of FIG. 1 stably oscillates at a resonance frequency of the dielectric resonator 27. Hereupon, though the resistor 26 is constituted to have the reflection coefficient .vertline..GAMMA..vertline..perspectiveto.0 in a frequency band of the used frequency, it is not always so for the frequency lower than or higher than the resonance frequency. For instance, when a dummy resistor comprising a 50.OMEGA. resistor and a 1/4 wavelength opened-ended strip line are used, the above-mentioned dummy resistor shows a characteristic of .vertline..GAMMA..vertline..perspectiveto.0 for the frequency range of lower than 1 GHz or 7-14 GHz when the LO frequency is 11 GHz, but is shows a characteristic of .vertline..GAMMA..vertline..perspectiveto.1 for the frequency range of 2-6 GHz. On the other hand, the FET 21 has a tendency to increase its gain as the frequency becomes lower, accordingly the reflection coefficient .vertline.S'.sub.11 .vertline. for a small signal seen from the gate terminal 24 towards FET 21 begins to display a negative characteristic to a low frequency, and accordingly .vertline.S.sub.11 '.vertline.&gt;1. Accordingly, even for the frequency range of .vertline..GAMMA..vertline..perspectiveto.1 except for the LO frequency, the oscillation condition of S.sub.11 .times..GAMMA..sub.R =1 can be satisfied, and this becomes the cause of a spurious oscillation, which is likely to be induced in a frequency range of 3-5 GHz.